The present invention relates to a pumping system for a vital signs monitor such as a blood pressure monitor or a gas analyzer.
Vital signs monitors such as blood pressure monitors and gas analyzers require pumps to pressurize a gas or other fluid In a blood pressure monitor the pump pressurizes a blood pressure cuff to occlude blood flow through an extremity of the patient. In a gas analyzer such as a capnometer the pump draws exhaled air from the patient through a sample chamber for analysis, as for example for infrared absorption analysis of exhaled gas concentrations. In both of these examples fluctuations in fluid pressure caused by the pump can interfere with the measurement.
In addition, it is often desirable or necessary to vary the pumping rate of the pump. For example, a capnometer pump is preferably adjustable to control flow rate through the sample cell In a blood pressure monitor the pumping rate is preferably adjustable to obtain the desired rate of change of pressure in the blood pressure cuff.
This invention is directed to an improved pump and pump controller that allow the frequency and pumping rate of the pump to be controlled in a particularly advantageous manner.
There have been a number of prior art pump controllers which supply an oscillating drive signal to a pump in order to control the pumping rate of the pump. Lovell, U.S. Pat. No. 3,162,134, Kofink, U.S. Pat. No. 3,221,798, and McGuire U.S. Pat. No. 3,610,782, all describe pumping systems in which a variable frequency drive signal is provided to a pump. The frequency of the drive signal is adjusted to modify the output rate of the pump as desired. Klochemann, U.S. Pat No. 3,819,305, discloses another type of pumping system which uses individual driving signals at a constant frequency. In Klochemann selected groups of pulses of the driving signal are blocked in order to provide a desired average throughput for the pump. Cuenoud U.S. Pat. No. 4,150,122 describes a micropipetting machine which supplies drive signals to a pump at a fixed frequency. This system counts the total number of pumping cycles in order to deliver the desired volume of liquid.
Woodward, U.S. Pat. No. 3,118,383, describes a pump system which includes a sensor that senses piston position in the pump This sensor automatically cycles the pump when the piston reaches an extreme of travel. In this way, a variable frequency drive signal is provided for the pump.
Of the systems described above, those of Woodward, Lovell Kofink McGuire all operate at variable frequencies. This can create a problem for a vital signs monitor, because pressure fluctuations associated with cycling of the pump can interfere with the vital signs measurement if the fundamental frequency of the pump is allowed to overlap the measuring frequency range of the vital signs monitor. Klochemann and Cuenoud operate pumps at fixed frequencies; however neither is intended for use with a vital signs monitor, and both would exhibit system drawbacks if an attempt were made to adapt them to a vital signs monitor. The approach taken in Klochemann of blocking sets of pulses of the drive signal to obtain an average pump throughput can result in excessive pressure variations. The Cuenoud micropipetter counts cycles to provide a desired dispensed volume, and is not well-suited to maintain a desired pressure.
The present invention is directed to an improved pumping system for a vital signs monitor that allows the pumping rate to be adjusted simply and reliably, that allows the pump to be operated at a fixed frequency well above the frequency range in which the measurement is made, and that allows the pumping rate to be controlled by varying the duty cycle of the drive signal while maintaining the fundamental frequency of the pump at a predetermined value.